in the x⁢yxyxy-plane. But cutting with z=0z0z=0 gives x2a2-y2b2= 0superscriptx2superscripta2superscripty2superscriptb2 0\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}\;=\;0, i.e. the pair of
linesy=±ba⁢xyplus-or-minusbaxy=\pm\frac{b}{a}x which is a degenerate conic.